372 Wli. J. V. ELSDEN ON THE AGE OE [Aug. I904, 



27. On the Age of the Llyn-Padarn Dykes. By James Vincent 

 Elsden, Esq., B.Sc. (Lond.), F.G.S. (Eead May 25th, 1904.) 



[Plate XXXII — Microscope-Sections.] 



I. Introduction. 



The characters of the basic sills of Caernarvonshire have been 

 described in detail by several writers, notably by Mr. Iiarker in his 

 well-known essay on the Bala Volcanic Series. There is a marked 

 absence of dykes in association with the outbursts of this period, 

 and the numerous basic dykes of this area have generally been 

 assigned to a later series of eruptions. The evidence upon which 

 this assumption rests is, however, not always satisfactory ; and 

 although Mr. Harker is inclined to favour their post-Carboniferous 

 age, he does not conceal the uncertainty of this conclusion in 

 several cases, and he adduces evidence which seems to point, at 

 least in some instances, to the possibility of the existence of more 

 than one group of these intrusions. 1 Dr. C. A Matley, also, finds 

 that in Northern Anglesey at least two groups of dykes occur, of 

 which the earlier are pre-Silurian and the later post-Ordoviciau. 3 

 But Mr. E. Greenly maintains that the later dykes of Anglesey, 

 including those of the Menai Straits, are certainly post-Carboni- 

 ferous, and may possibly be even of Tertiary age. 3 



With regard to the Llyn-Padarn dykes, with which this paper is 

 chiefly concerned, no very detailed description appears to have been 

 published, although several authors have incidentally referred to 

 them, as will be mentioned hereafter. It appears to have been 

 generally assumed that these dj r kes are of the same age as those of 

 the Menai Straits, to which they are supposed to bear a general 

 resemblance, both in petrographical character and in direction. 

 With regard to the former, however, this is by no means the case. 



In the present paper it is proposed to examine this question in 

 detail, and to produce evidence which seems to suggest that the bulk 

 of the ' greenstone "-dykes of this area belong to an earlier period 

 of eruption than has been generally assigned to them, and there is 

 proof that some of these ' greenstones ' may even be older than the 

 quartz-felsite of the Llyn-Padarn ridge. The greater part, however, 

 if not actually of Bala age, seem to have been intruded before 

 the great post-Bala crust-movements, which produced the folding 

 of the Lower Cambrian rocks of Llanberis, had entirely ceased. 

 At the same time, the evidence does not exclude the possibility that- 

 some of the intrusions may be of a later date. 



The evidence upon which these conclusions rests is based mainly 



1 ' On some Anglesey Dykes" Geol. Mag. 1887, p 409 ; & ibid. 1888, p. 267. 



2 'Geology of Northern Anglesey' Quart. Journ. Geol. Soc. vol. lvi (1900) 

 p. 249. 



3 ' On the Age of the Later Dykes of Anglesey ' Geol. Mag. 1900, p. 160. 



